1. Field of the Invention
The disclosure generally relates to offshore floating structures. More particularly, the disclosure relates to large, multi-sectional, offshore floating structures that are fabricated and then assembled while floating in water.
2. Description of the Related Art
A spar platform is a type of floating offshore structure typically used in very deep waters and is among the largest offshore platforms in use. A spar platform includes a large cylinder or hull supporting a typical rig topsides. The hull does not extend all the way to the seafloor and typically rely on a traditional mooring system to maintain their position. The spar platform can further include a truss structure of a generally open construction that is disposed below the floating hull. The truss can support risers and provide stability for the hull. The combination has been termed a “truss spar hull” platform. Typically, about 90% of the platform is underwater. The large hull and/or truss serves to stabilize the platform in the water, and allows movement to absorb the force of potential high waves, storms or hurricanes.
An exemplary truss spar hull platform can be about 250 meters long and about 45 m in diameter. The size of some such platforms has traditionally been limited by the capacity of vessels to haul the platform. There has been a desire to manufacture a larger platform than the vessels are capable of carrying.
FIG. 1 is a side schematic view of a prior art truss spar hull with an additional temporary float tank using a method to couple a truss with a spar hull. In at least one prior art example, a truss spar hull platform 2 was produced by fabricating a hull 4 in one fabrication yard and a truss 6 in different fabrication yard. The truss 6 included four (4) legs 10 with bracing 12 therebetween. The hull 4 was hauled by a vessel to the truss fabrication yard and offloaded into a quay adjacent the fabrication yard. The hull 4 with its traditional float compartments was floated on its side in the quay with a water level 8 above the seabed 20. On their sides, the hull and truss extended about 14 stories high. The open structure truss 6 used a temporary float tank 14 and a float tank 16 with wing tanks extending outward from the truss, the tanks having multiple chambers to adjust and ballast to help align the floating truss with the floating hull. The truss 6 was floated in the quay with the temporary float tank 14 toward an upper end of the truss and a float tank 16 toward a lower end of the truss. Special cofferdams 22, each weighing about 35 tons, were lowered into the water, sealed around the legs 10 and other components that were underwater, and pumped dry to allow welding therein. The temporary float tank 14 and the float tank 16 were ballasted to adjust an alignment of the legs 10 in conjunction with a special mating guides 24 on the truss legs 10 to corresponding mating portions 26 on the hull 4. The truss legs 10 and bracing 12 were welded to the hull 4. The welding was done sequentially on different portions of the two sections, because the different portions required different alignments for proper welding precision. Other special fabrication and ballasting techniques were used to connect the two sections. After the welding, the remaining components for the offshore structure were assembled to the combined structure and the entire structure floated to an offshore installation site.
Thus, while the fabrication process showed that the truss and hull could be made separately and assembled while floating to complete fabrication of such a large offshore structure, the process was costly and intricate. The fabrication was considerably challenging to align the large components and maintain alignment and dimensional control during the welding and in underwater conditions using the cofferdams.
There remains a need for an improved system and method of assembling multi-sectional truss spar hull platforms.